Coated electrical apparatus and method of making the same



May 8, 1956 G. R. RADLEY 2,744,839

COATED ELECTRICAL APPARATUS AMD METHOD oF MAKING THE SAME Filed Aug. 24, 1955 United States Patent O COATED ELECTRICAL APPARATUS AND METHoD or MAKING THE SAME Guy R. Radley, Whitefish Bay, Wis., assignor toCutler- Hammer, Inc., Milwaukee, Wis., a corporation of Delaware Application August 24, 1953, Serial N o. 376,039

4 Claims. (Cl. 117218) This invention relates to improved methods of. and means for protecting electrical apparatus frommoisture and mechanical damage.

While not limited thereto, the invention is especially useful for protecting electrical windings and terminal connectors of electrical apparatus.

A primary object of the invention isto provide iinproved methods and means for protecting electrical apparatus against mechanical damage and the harmful effects of moisture.

Another object is to provide, for electrical apparatus, improved coatings which have long life, are resistant to mechanical abrasion, and are impervious to moisture.

Another object is to provide coatings of the. aforedescribed character which readily dissipate heat.

Another object is to provide. improved wire-Wound electrical resistors having protective coatings which` will readily dissipate heat and will afford, for the resistance windings and for the connections betweenthe windings and the resistor terminals, increased protection against corrosion and mechanical damage..

Various other objects and advantages of the invention will hereinafter appear.

The accompanying drawings illustrate certain embodiments of the invention and the steps of producing. the same, which will now be described, it being understood that other embodiments are possible without departing from the spirit or scope of the invention as dened-in the appended claims.

In the drawings, Figure l is an elevational View of an electrical resistor unit which is representativek of one of the classes of electrical apparatus to which the invention is applicable;

Fig. 2 is an enlarged, fragmentary, sectional view illustrating one step in the production ofV a coating cmbodying the invention as applied toaI resistor unit;

Fig. 3 is an enlarged, fragmentary, sectional view illustrating a part of a resistor unit provided with a protective coating embodying the invention;

Fig. 4 is an enlarged, fragmentary,. sectional View illustrating one step in the production of a coating. of alternative form embodying the invention as applied to ay resistor unit; n

Fig. 5 is an enlarged, fragmentary, sectional View illustrating a part of a resistor unit provided with a protective coating of alternative form embodying the invention;

Fig. 6 is an elevational view of a resistor unit provided with a coating embodying the invention; and

Fig. 7 is an enlarged, fragmentary, View of a part of a resistor unit showing a preferred method of connecting resistance wire to an end terminal.

In addition to being made safe and shock-proof, certain types of electrical apparatus must be protected against corrosion, electrolytic action, mechanical vibration, abrasion and the like. Of these types, certain ones have e ICC highv operating temperatures and are required to dissipate substantial amounts of heat. While not limited theretothis invention is. particularly useful. in protectingl apparatus of the latter type. Because wire-wound resistors arev subjectedy to the most severe of these conditions, the application of the invention to such resistors will be described, it being understood that the application and-usefulness of the invention is not limited thereto.

A representativeform of wire-Wound resistor is shown in Fig. l. Itv consists of aninsulating base 10, a length of resistance wire 12 coiledthereabout and a pair of ferrule-like end connector members14 and 16; Each end of wire 12 is fastened to oneoffa pair yof short pieces 1.3-, 13. of wirewhichare tucked under a respectively associated one of'. the endconnector members 14y and 1'6; The connector members or terminals are clamped or spun against the. basel 10 and said pieces of Wire 13 so that. they have goodv electrical contact with the latter. lnnormal-use the uncoated resistance wire 12 expands and may become loose enough to permit adjacent turns of the wire to touch one: another. Merely-coating the unit with an adhesive material is not an adequate remedy,

`for if such material is of sufficiently low viscosity for uniform application thereof, itsfs'urface tension will cause it to flow away from the upper surfaces of the wire. Suchupper surfaces will` thereafter be unprotected against abrasion and other mechanical damage and, ifl touched, may result in serious electrical shock. Also, if the coating4 becomes crazed, moisture will. be admitted and will corrod'e. the wireand` may forman electrolyte with, certain chemical constituents of the base 10 or of the coating and eiect electrolytic action between the turns of the resistance wire and also between the resistance wire-and the endl connector members 14 and 16. The aforedescribed difliculties and others are overcome by applicants invention in which the apparatus to be pro` tected is first provided with. a layer 18 (Fig. 2) of a mixture of adhesive material 20 and solid filler material 22. The adhesive materialisthen hardened. A second layer 24 of adhesive material is applied'over the rst'layer (see Fig. 3) and is hardened.

The iller material 22L inthe rst layer 18 of adhesive material gives body to said layer to permit its even application to the apparatus to be protected by spray-ing, brushing, or dipping methods and so that the effect of surface` tension is substantially overcome. Wires, suchv as wire 12, and the'like, areV substantially covered by adhesive and filler material. After the adhesive material ishardened, the ller material `acts to inhibit crazing of the layer.

Protection ofthe apparatus against moisture and protectionof the coating against heat isfafforded by the use ofl waterproof and heat resistant adhesive and filler materials. I prefer to employ a resinous adhesive materiai such as silicone resin and a iillerY comprising grains. of quartz. Approximately equal volumes of Dow Corning number 802y silicone resin. andDoW Corning number 803 silicone resin, towhich isadded enough solvent such as toluol so that the resulting mixture has a specific gravity about equal to that of Water, provide a satisfactory liquid adhesive mixture. When froml one-fourthl to two-thirdvsvot a part of quartz: grains. about one mil in diameter are added to one pait of the liquid, the resulting mixture isl one which may be applied to articles by dipping, brushing, or spraying methods. I'

This layer is then air dried until a skin forms on the outer surface. The drying time depends largely upon the composition of the adhesive material. Usually drying for aboutv one hour is sufficient. The layer is then baked to hardness beginning at a low temperature. The temperature is increased gradually or may be increased in steps over a period of hours. The maximum temperature is usually that which the base material can withstand without warping or blistering or otherwise being damaged. It is desirable that the nal baking temperature be about 500 Fahrenheit when the characteristics of the base material permit.

The resulting layer is very strong and tough and will not craze at temperatures as high as 700 F. The space between the resistance wires is substantially filled. However, the surface may be somewhat rough and certain portions of the resistance wire may not be sufficiently covered. A second layer 24 of adhesive material, preferably comprising the same material as the first layer 13 but not including ller material, is applied over said first layer. A coloring agent or coloring matter may be mixed with the second layer of adhesive 24 before it is applied to improve the heat dissipating qualities of the protective coating. Dark colors are preferred. Black is best because it radiates heat at a greater rate than other colors so that the resistor can be given a higher watt rating when its coating is colored black. The addition of powdered magnetic iron oxide to the adhesive material will color the mixture black. This material will retain its color, even when subjected to the maximum safe operating temperature of resistors, and has good dielectric strength.

The second layer 24 is then hardened in the same manner as was the rst layer 18. The resultant coating has a pebbled outer surface which withstands higher operating temperatures than does a smooth surface composed of the same materials. This coating has unusually good heat dissipation characteristics, a very tough, hard, waterproof and heat resistant inner layer and a relatively less hard outer layer having expansion characteristics like that of the inner layer. The resinous adhesive material adheres tenaciously to the base material and to the wires so that the protective coating is not lifted away from the base by expansion of the wire.

In certain instances, as where the resistance wire 12 is of large diameter, it is desirable to sprinkle or otherwise apply larger particles 26 of ller material (see Fig. 4) on the iirst layer of adhesive material 20 and filler material 22 before said first layer is air dried or baked. The larger particles 26 of filler material are permitted to embed themselves in the adhesive material 2@ and may be further covered by the application of additional quantities of the mixture of adhesive material 20 and small grains of ller material 22. The resultant layer 28 is then air dried and baked in the manner aforedescribed. As shown in Fig. 5, a second layer fit1 of adhesive material, which may be mixed with coloring matter, is then applied over layer 28 and is air dried and baked.

The large particles of ller material 26 are preferably egg-shaped and have smooth surfaces so that the upper surface of layer 28, while pebbled, is relatively smooth, and so that layer 30 will be of relatively uniform thickness. l now prefer to employ'particles, ranging in size from about to 10 mils diameter, of a sand which comprises quartz and Zircon. Such sand is found along portions of the shores of the Gulf of Mexico and is commercially available under the trade name Zircon A resistor of the type illustrated in Fig. 1, to which a protective coating 32 has been applied, is illustrated in Fig. 6. The protective coating 32 extends partly over the end connector members or terminals 14 and 16 of the resistor so that the points at which the resistance wire is tucked under and connects to the respective terminals are completely covered. Thus entry of moisture at said points and subsequent electrolytic action between the wire and the terminals which would result in oxidation of the resistance wire and, consequently, poor electrical connections between the wire and terminals, is prevented. Because of the protection against moisture which is aiforded by the coating, it is possible to connect the resistance wire to the resistor terminals without soldering, welding or other expensive and troublesome procedures. As illustrated in detail in Fig. 7, a short length of wire 13, preferably of the same metal as the resistance wire 12, is firmly clamped between the terminal 14 and the base 10 by the process known as spinning. The end of the resistance wire 12 is twisted with the short wire 13 and the twisted connection is thereafter bent down against the base 10. The protective coating is applied over the twisted joint and over the spun end of ferrule 14 to protect the twisted joint from deterioration.

vl claim:

l. In producing a protective coating on electrical apparatus, the steps of applying a mixture comprising one part of liquid silicone resin adhesive material and from one-fourth to two-thirds of a part of particles of heat resistant mineral ller material having a diameter of about one mil, to the surface to be protected, embedding particles of substantially smooth surfaced heat resistant mineral iiller material having a diameter of approximately tive to ten mils in the mixture of adhesive material and particles of ller material, air drying and baking said adhesive material to harden the same, applying a second layer comprising a mixture of liquid silicone resin adhesive material and dark coloring matter over the rst layer of filler materials and hardened adhesive, and thereafter air drying and heating said second layer to harden the same.

2. The subject matter of claim 1 wherein the dark coloring matter is black and of good dielectric strength.

3. An article comprising electrical apparatus having thereon a protective coating consisting of a iirst layer comprising a mixture of one part of hardened silicone resin and from one-fourth to two-thirds of a part of particles of heat resistant mineral iiller material and having particles of approximately ve to` ten mils diameter of substantially smooth surfaced and heat resistant mineral iller material embedded therein, and a second layer bonded to said first layer and comprising a mixture of hardened silicone resin and coloring matter.

4. The subject matter of claim 3 wherein the coloring matter is black and of good dielectric strength.

References Cited in the file of this patent UNTED STATES PATENTS 2,093,501 Williams Sept. 2l, 1937 2,229,985 Nowak et al Ian. 28, 1941 2,258,218 Rochow Oct. 7, 1941 2,290,905 Butler July 28, 1942 2,330,365 Jackson Sept. 28, 1943 2,459,018 DeMonte et al Jan. 11, 1949 2,523,037 Mathes Sept. 19, 1950 2,523,065 Sage Sept. 19, 1950 2,531,169 Sprung Nov. 21, 1950 2,664,364L Thom Dec. 29, v1953 

1. IN PRODUCING A PROTECTIVE COATING ON ELECTRICAL APPARATUS, THE STEPS OF APPLYING A MIXTURE COMPRISING ONE PART OF LIQUID SILICONE RESIN ADHESIVE MATERIAL AND FROM ONE-FOURTH TO TWO-THIRDS OF A PART OF PARTICLES OF HEAT RESISTANT MINERAL FILLER MATERIAL HAVING A DIAMETER OF ABOUT ONE MIL, TO THE SURFACE TO BE PROTECTED, EMBEDDING PARTICLES OF SUBSTANTIALLY SMOOTH SURFACES HEAT RESISTANT MINERAL FILLER MATERIAL HAVING A DIAMETER OF APPROXIMATELY FIVE TO TEN MILS IN THE MIXTURE OF ADHESIVE MATERIAL AND 